JEE Chemistry Environmental Chemistry Notes Set 02

Pollutants and the Atmosphere

Environmental pollution is any undesirable change in our surroundings, caused by human activity or natural events, that harms plants, animals or humans. A pollutant is a substance present in a concentration higher than its natural level. Some pollutants are quickly degraded by natural processes (for example vegetable waste); others are non-degradable and persist for decades (DDT, plastics, heavy metals, nuclear waste).
The atmosphere is a blanket of gases held to the earth by gravity. Its total mass is about \( 5 \times 10^{15} \) tonnes. From the ground upward the layers are the troposphere (pollution by smog, acid rain, greenhouse gases), the stratosphere (ozone layer), the mesosphere and the thermosphere. The troposphere extends to about 10 km.

Oxides of Sulphur and Nitrogen

Sulphur in fossil fuels burns to SO\(_2\), and some is oxidised further to SO\(_3\) (particulates act as catalysts). \( \ce{SO2 + O3 -> SO3 + O2} \) and \( \ce{SO2 + H2O2 -> H2SO4} \). SO\(_3\) with moisture gives sulphuric acid. Effects of SO\(_2\): asthma, bronchitis, eye irritation, and at high concentration stiffness and falling of flower buds.
Oxides of nitrogen form when N\(_2\) and O\(_2\) combine at high temperature in engines: \( \ce{N2 + O2 -> 2NO} \) (about 1483 K), then \( \ce{2NO + O2 -> 2NO2} \). NO\(_2\) causes the reddish-brown haze, damages leaves and textiles, and is toxic to tissue. Lightning also makes NO; the nitrate formed acts as a natural fertiliser.
Unburnt hydrocarbons are carcinogenic and cause ageing and shedding of leaves in plants.

Oxides of Carbon

Carbon monoxide comes from incomplete combustion and vehicle exhaust. It binds haemoglobin to give carboxyhaemoglobin, about 300 times more stable than oxyhaemoglobin. Blood then carries less oxygen, causing headache, weak eyesight and nervousness; high levels harm pregnancy.
Carbon dioxide (about 0.03 percent of air) comes from respiration, fossil fuels, limestone decomposition in cement making and volcanoes. Photosynthesis removes it, but deforestation and burning fuels raise its level.

Greenhouse Effect and Global Warming

Visible sunlight warms the surface, which re-emits infrared radiation. Greenhouse gases are transparent to sunlight but absorb infrared, so part of the heat is trapped. This natural effect keeps the earth habitable; excess greenhouse gases cause global warming. CO\(_2\) is the main contributor. The others are methane (paddy fields, coal mines, rotting waste), nitrous oxide (fertilisers, fossil fuels), CFCs (man-made) and ozone and water vapour.
Consequences: melting of polar ice, rise in sea level and spread of diseases such as malaria and dengue.

Acid Rain

Rain water is naturally acidic with pH about 5.6 because \( \ce{CO2 + H2O <=> H2CO3 <=> H+ + HCO3-} \). When the pH falls below 5.6 it is acid rain. The main causes are SO\(_2\) and NO\(_2\) from fossil fuels, volcanoes and forest fires.
\( \ce{2SO2 + O2 + 2H2O -> 2H2SO4} \) and \( \ce{4NO2 + O2 + 2H2O -> 4HNO3} \). Acid rain washes nutrients from soil, harms aquatic life, corrodes pipes (releasing Fe, Pb, Cu into water) and attacks marble: \( \ce{CaCO3 + H2SO4 -> CaSO4 + CO2 + H2O} \). This is why the Taj Mahal marble is slowly discoloured.

Particulate Pollutants

Particulates are tiny solid or liquid particles in air. Viable particulates are living (bacteria, fungi, moulds, algae). Non-viable particulates are of four kinds: smoke (from combustion), dust (solid particles over 1 micron made by crushing and grinding), mists (spray droplets or condensed vapour, such as H\(_2\)SO\(_4\) mist) and fumes (condensed vapours of metals, metal oxides or solvents).
Particles larger than about 5 microns stay in the nose; particles near 1 micron enter the lungs. Leaded petrol was the main source of air-borne lead, which damages red blood cells.

Classical and Photochemical Smog

Smog is smoke plus fog. The two types differ as follows.

Mechanism of photochemical smog: \( \ce{NO2 ->[h\nu] NO + O} \), then \( \ce{O + O2 -> O3} \), and \( \ce{NO + O3 -> NO2 + O2} \). Ozone and NO\(_2\) react with unburnt hydrocarbons to give formaldehyde, acrolein (CH\(_2\)=CH-CHO) and peroxyacetyl nitrate (PAN, an eye irritant that damages plants and cracks rubber).
Control: catalytic converters in vehicles, planting trees such as pinus, juniperus, quercus, pyrus and vitis that metabolise NO, and spraying compounds that generate free radicals which remove the smog radicals.

Stratospheric Ozone and the Ozone Hole

Ozone forms in the stratosphere: \( \ce{O2 ->[UV] 2O} \) and \( \ce{O + O2 -> O3} \). The layer absorbs harmful UV of about 200-315 nm, which would otherwise cause skin cancer, cataract and damage to plankton and plants.
CFCs (freons) are inert, non-toxic and non-flammable, so they are used in refrigerators, air conditioners and foam. In the stratosphere UV splits them: \( \ce{CF2Cl2 ->[UV] Cl. + .CF2Cl} \). Then \( \ce{Cl. + O3 -> ClO. + O2} \) and \( \ce{ClO. + O -> Cl. + O2} \). Chlorine is regenerated, so one atom destroys many ozone molecules.
Over Antarctica, in winter polar stratospheric clouds form. \( \ce{ClO + NO2 -> ClONO2} \) and \( \ce{Cl + CH4 -> HCl + CH3} \) normally act as chlorine sinks. On the cloud surfaces \( \ce{ClONO2 + H2O -> HOCl + HNO3} \) and \( \ce{ClONO2 + HCl -> Cl2 + HNO3} \). In spring sunlight splits HOCl and Cl\(_2\) into chlorine atoms and ozone is rapidly destroyed.

Water Pollution

Sources may be point sources (discharge pipes) or non-point sources (farm runoff, storm drains). Major pollutants are pathogens (from sewage), organic wastes, plant nutrients (fertilisers), toxic metals, sediments, pesticides, radioactive substances and heat.
Dissolved oxygen (DO) in cold water is up to about 10 ppm; fish growth suffers below 6 ppm. Biochemical oxygen demand (BOD) is the oxygen needed by bacteria to break down the organic matter in a given volume of water. BOD below 5 ppm means clean water; 17 ppm or more means highly polluted water.
Eutrophication: nutrients (especially phosphates) cause a dense algal bloom; when the algae die and decay the dissolved oxygen drops, killing fish and reducing biodiversity.

Drinking Water Standards

Soil Pollution, Industrial and Domestic Waste

Pesticides are largely water-insoluble and non-biodegradable, so they pass up the food chain and build up in higher animals. Insecticides include DDT, BHC and malathion (DDT is banned in India). Herbicides include sodium chlorate, sodium arsenite and the organic triazines.
Industrial waste may be biodegradable (cotton mills, food units, paper mills) or non-degradable (fly ash, blast-furnace slag, red mud, gypsum). Fly ash and slag are used in the cement industry. Domestic waste is segregated: biodegradable waste goes to landfills and becomes compost; non-biodegradable waste (plastics, glass, metals) is recycled.

Green Chemistry

Green chemistry designs processes that reduce material use, energy use and waste, and avoid hazardous substances. Water is preferred as a solvent because it is cheap, non-flammable and non-carcinogenic.
Examples: dry cleaning with liquefied CO\(_2\) and a detergent instead of tetrachloroethene; bleaching paper with H\(_2\)O\(_2\) and a catalyst instead of chlorine; making ethanal from ethene in one step with a Pd(II)/Cu(II) catalyst in water, in about 90 percent yield.